isotopically modified compound CHEBI:139358 Any molecular entity in which the isotopic ratio of nuclides for at least one element deviates measurably from that occurring in nature. The term includes both <em>isotopically substituted</em> compounds (in which essentially all the molecules of the compound have only the indicated nuclide(s) at each designated position) and <em>isotopically labeled</em> compounds (a formal mixture of an isotopically unmodified compound with one or more analogous isotopically substituted compound(s). molecular entity